1. Field of the Invention
The present invention relates to a system and method for increasing the fuel economy of a steering system.
2. Description of Related Art
In conventional power assist steering systems using hydraulic fluid to provide steering assist, the pump provides a high outlet flow even if no steering assist is required. This will result in a high pump drive torque demand from the engine accessory drive. This drive torque demand directly affects vehicle fuel efficiency. Additionally, the high fluid flow results in poor road feedback and steering feel at high vehicle speeds leading to uncomfortable drive performance.
Prior art systems use an electronic flow control device to control the hydraulic fluid flow from the power steering pump to the steering gear. Such a system is described in U.S. Pat. No. 4,473,128. However, the fluid flow is not controlled for fuel efficiency. Furthermore, analog circuitry is used for the generation of control signals for the electronic flow control device.
An alternative implementation with an electronic flow control device is described in U.S. Pat. No. 4,629,025. Fluid flow is controlled in response to variations in steering wheel rate. However, fluid flow is not controlled to optimize fuel efficiency.
Implementations of the required hardware components for a system with an electronic flow control device are described in U.S. Pat. Nos. 5,147,183, 5,261,637 and 5,111,660.
U.S. Pat. No. 5,029,660 describes an implementation of a steering system using an electronic flow control device, which is designed to provide variable fluid flow for different driving situations. However, the algorithm is not designed to provide fuel efficiency.
An implementation for a steering system with variable fluid flow is described in U.S. Pat. Nos. 6,069,460 and 6,101,435. Here an electro-hydraulic system is described. The electro-hydraulic system uses an electric motor to drive the power steering pump as opposed to the power steering pump being driven by the engine accessory drive. The algorithm described is for the speed control of the electric motor as opposed to control of an electronic flow control device, such as a valve.
In U.S. Pat. No. 6,098,741 and EP 1 024 072 A1 a system with an electronic flow control device is described. However here a steering torque sensor is required to provide variable fluid flow.
In view of the above it is apparent that there is a need for an improved system and method to control a vehicle steering system.